Disposable endoscopes are used once and then discarded. Disposable endoscopes are becoming increasingly significant for medical applications because the repeated sterilization of the critical components, which come into direct contact with the patient, is eliminated. An important criterion for disposable endoscopes is that a simple and low-cost optic be provided in the endoscope tube. At the same time, however, a high imaging performance of the endoscope system is required. Above all, this is characterized by the transmission or brightness and an optimal correction of aberrations. At the same time, and depending upon the area of application, tube lengths of between 200 and 600 mm with a tube diameter of less than 10 mm are desirable.
Disposable endoscopes are described, for example, in U.S. Pat. Nos. 5,188,092 and 4,964,710 as well as in PCT publication WO 93/15647. For low-cost optics, plastic lenses are used exclusively in the endoscope tube as described in PCT publication WO 93/15647. Very considerable limitations occur with respect to the correction of imaging errors in this context because the selection of available materials is minimal at the present time. Furthermore, plastic lenses do not yet satisfy the requirements with respect to the quality of the refracting surfaces and the homogeneity of the medium. In the endoscope disclosed in U.S. Pat. No. 5,188,092, these present-day problems are avoided by utilizing pressure molded glass lenses. Furthermore, a simplified assembly technique is suggested.
These known optical systems are, however, very complex. Several multi-lens intermediate imaging systems are always required in order to achieve the tube length desired for medical applications. In one embodiment of the endoscope disclosed in PCT publication WO 93/15647, a double-lens objective is, for example, provided at the distal end of the endoscope tube and three intermediate imaging systems are arranged within the endoscope tube. Each of the intermediate imaging systems includes six lenses. Even when each individual lens itself is relatively inexpensive in comparison to the glass lenses classically produced by grinding and polishing, the total number of the lenses mounted in the endoscope tube lead, however, to a considerable cost of the exchangeable component.
Endoscopes have already been proposed wherein a CCD-camera chip is mounted at the distal region of the endoscope tube directly behind the endoscope objective. The viewing field is then imaged on the CCD-camera chip without previous intermediate imaging. In this way, a significantly simpler optical configuration is achieved because the optical image does not have to be guided over a longer path. Such an endoscope is, for example, disclosed in U.S. Pat. No. 4,757,805. This apparatus concept is, however, unsuitable for a disposable endoscope because the rather expensive CCD chip is included in the disposable parts.
Published German patent application 4,207,092 discloses an endoscope containing the following in the endoscope tube: a double-lens objective, two intermediate imaging systems each of which is configured as a rod lens and an image collimating lens. This published patent application, however, provides no optical data whatsoever or information as to the length of the endoscope tube.
U.S. patent application Ser. No. 08/100,276, filed Jul. 28, 1993, discloses a stereoendoscope including a distal front group having a negative diffractive power. The stereoendoscope, together with an intermediate group having a positive diffractive power, generates a real image of the object. However, here too the first real intermediate image of the object lies already at a distance of approximately 150 mm behind the front group. In order to achieve a tube length desired for endoscopy, it is here therefore also necessary to still provide at least a second intermediate image within the endoscope tube.